Allow tokenUrl to be relative

Looks like the change before:
15c2fd5#diff-1a1a2865f336a376229eaffe0f56caac01c7a3a77602e07fe4a38fd550c6c318

Has been reverted:
0101a03#diff-6676609c9c37705f6953ffcc9eb6d33164df50f241d4844c5665cf5ff4305995R16

As mentioned, relative URLs are supported in OpenAPI 3.0+:
swagger-api/swagger-ui#5243

Author: Fokko

openapi_python_client/schema/openapi_schema_pydantic/oauth_flow.py

@@ -13,7 +13,7 @@ class OAuthFlow(BaseModel):
     """
 
     authorizationUrl: Optional[AnyUrl] = None
-    tokenUrl: Optional[AnyUrl] = None
+    tokenUrl: Optional[str] = None
     refreshUrl: Optional[AnyUrl] = None
     scopes: Dict[str, str]